1. Field of the Invention
The invention concerns a method of monitoring and providing protection against electrical arcs, implemented in a high-tension voltage generator device and in particular a device of this kind forming part of an installation for electrostatic spraying of a coating product, such as paint or heat-meltable powder. The invention also concerns a high-tension voltage generator device incorporating means for implementing this method.
2. Description of the Prior Art
In an installation of the type mentioned hereinabove, the paint divided into fine droplets is generally charged to a high voltage before leaving the sprayer and is directed towards the object to be painted, which is electrically grounded.
Due to variations in certain parameters, such as the value of the high-tension voltage, the shape of the object and its distance from the sprayer, electrical arcs may be produced which can cause significant damage, especially if the product is flammable.
A high-tension voltage generator device of this kind generally comprises means forming a low-tension direct current voltage supply, a high frequency direct current to alternating current converter fed by the low-tension direct current voltage supply (this is generally an oscillator, a chopping circuit or some other analogous device), a voltage step-up transformer and a voltage rectifier-multiplier of conventional design comprising a cascade circuit of diodes and capacitors. The primary winding of the transformer is fed by the converter and the secondary winding is connected to the low-tension input of the voltage rectifier-multiplier. These various components are divided between a fixed subsystem and a mobile subsystem, the latter forming the paint sprayer that is manipulated by an operator or fixed to a robot arm, for example. The two subsystems are generally interconnected by a harness comprising a plurality of electrical conductors and the paint feed line. The electrical conductors include a so-called "return conductor" connecting one pole of the low-tension input of the voltage rectifier-multiplier to an impedance (generally a high value resistor) that is electrically grounded in the fixed subsystem. This provides a simple way to measure and "monitor" the value of the high-tension current and variations in it which make it possible to predict an imminent electrical arc. The measurement of this current is used, for example, to control a voltage limiter circuit in the low-tension direct current voltage supply. The other conductors include two wires connecting the low-tension supply and the direct current to an alternating current converter if the latter is in the mobile subsystem or two wires connecting the output of the direct current to alternating current converter to the primary winding of the voltage step-up transformer if the converter is in the fixed subsystem. In the former case, the two wires carry a direct current voltage and in the latter case they carry a so-called "high-frequency" (in the order of 20 to 25 kHz) alternating current voltage.
Electrical arcs may also be caused by deterioration of one of these conductors. It has already been proposed to use specific means to monitor the continuity and the insulation of the conductors connecting said fixed and mobile subsystems.
The invention proposes to carry out such monitoring by simple, effective and inexpensive means exploiting the fact that the stray capacitance of the voltage step-up transformer normally injects a high-frequency residual component into the return conductor, so that this residual component can be sensed and measured in the fixed subsystem. It has been observed that, whatever type of defect affects any of the conductors mentioned above, it causes the stray component to disappear or its level to drop significantly. The invention exploits this phenomenon.